Semiconductor gas sensors which can be used even in a high temperature corrosive atmosphere have been known and used for detecting poisonous gases under severe conditions (see "Chemical Sensor Technology", vol. 1, Kodansha and Elsevier (1988), pp 1-13, 15-38). This semiconductor gas sensor utilizes a principle that electrical conductivity on a surface of a semiconductor is changed by loading and unloading of electric charge which is carried out when chemical species are adsorbed onto and desorbed from the surface of the semiconductor (see Anal. Chem. 34 1502 (1962) "Adsorption and Desorption Behavior of Oxygen on Semiconductive Metal Oxides", Denki-Kagaku. 49. 367-368 (1981)). This semiconductor gas sensor is liable to react with various types of reducing gas and accordingly provides only an insufficient ability in discrimination of a plurality of reducing gases from one another. Catalysts have been used to improve this gas discriminating ability of the gas sensor (see Bull. Ceram. Soc. Jpn, 11,205 (1976), Nihon Kagaku-Kaishi, 1591 (1980). However, even though the catalysts are used, it has been difficult to discriminate, for example, hydrogen gas from carbon monoxide gas and therefore it has been considered that an important subject is to improve the ability for discriminating a gas to be measured (see "Ceramic Sensors", Kodansha (1984)).
A heterojunction type gas sensor which uses the p-n junction of the semiconductor has been studied to develop a gas sensor with a high gas discriminating ability. The sensing performance of this heterojunction type gas sensor to steam, combustible gases (for example, hydrogen, carbon monoxide gas, and propane), chlorine gas and nitrogen oxide gas has been investigated and studied (see "The Current Voltage Characteristics of CuO/ZnO Heterojunctions", Nihon-Kagaku-Kaishi, 1154-1159 (1985); "The Detection of Carbon Monoxide by the Oxide-Semiconductor Hetero-Contacts", Nihon-Kagaku-Kaishi, 477-483 (1987); "Selective CO Gas Sensing Mechanism with CuO/ZnO Hetero-contact", J. Electrochem. Soc., Vol. 137, No. 3, 940-943 (1990); "Effects of Interface States on Ga Sensing Properties of a CuO/ZnO Thin Film Heterojunction" (in printing); and "Intelligent Ceramics", Ferroelectrics, Vol. 102, 251-257 (1990)). These studies are intended to utilize a phenomenon that a value of a current which flows through the heterojunction is affected by an ambient atmospheric gas and measure a kind and a content of gas based on a voltage-current relationship obtained by applying a DC voltage to the heterojunction. These studies have reported that a heterojunction type gas sensor which provides a high sensitivity to carbon monoxide gas and hydrogen gas (particularly, carbon monoxide gas) can be made by combining cupric oxide (CuO; hereafter referred to as "copper oxide"), which is manufactured with basic copper carbonate as raw material, and zinc oxide (ZnO) (see "The Detection of Carbon Monoxide by the Oxide-Semiconductor Hetero-Contacts, Nihon-Kagaku-Kaishi, 477-483 (1990)). According to this report, it is described that the sensitivity of the heterojunction type gas sensor to carbon monoxide gas and hydrogen gas is raised when a forward biased specific voltage (0.5 V) is applied to the heterojunction and, though carbon monoxide gas cannot be discriminated from hydrogen gas only with this biasing, a CO density can be determined by calculation based on a difference between the sensitivity when the 0.5 V voltage is applied and the sensitivity when the other voltage (for example, 1.0 V) is applied if it is known in advance that two kinds of gases, that is, carbon monoxide gas and hydrogen gas are contained.
In these studies using the heterojunction type gas sensor, gases are discriminated based on the difference of the values of current which flows in the heterojunction when a certain specified voltage is applied to the heterojunction. However, this difference of the current value is extremely small and therefore some improvement is required for distinct discrimination of gases.
There have been several views as evaluation methods with respect to a sensing performance using DC characteristics, static capacity and impedance of a single ceramics semiconductor and those methods which can be used for the gas sensors (see "Properties of Polycrystalling Gas Sensors Based on d. c. and a. c. Electrical Measurements", Sensors and Actuators B, 8, 231-235 (1992); "Application of Mixed Oxide Capacitor to the Selective Carbon Dioxide Sensor", J. Electrochem, Sec., Vol. 138, 173-176 (1991) [11200]; and "Application of Mixed Oxide Capacitor to the Selective carbon Dioxide Sensor", J. Electrochem, Soc., Vol. 139, 2881-2885). However, examinations for using as the gas sensors which excel in gas discrimination performance are insufficient.